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Soil Moisture Active Passive (SMAP)


Launched on 31 January 2015, the NASA Soil Moisture Active Passive (SMAP) mission is designed to principally measure soil moisture and freeze/thaw state from space for all non-liquid water surfaces globally within the top layer of the Earth. The mission additionally provides a value-added Level 4 terrestrial carbon dataset derived from SMAP observations.

SMAP is now also building upon the legacy of Aquarius/SAC-D mission in delivering both soil moisture and derived sea surface salinity (SSS) observations for the world’s oceans. With the loss of the Aquarius mission on 7 June 2015 it became critical also to continue the time series of global salinity observations important to studies of the Earth’s water cycle.  Because both Aquarius and SMAP shared a L-band feed-horn configuration, lessons learned from the algorithm development under the Aquarius mission could be applied to SMAP to retrieve SSS via SMAP. However, because of the larger swath coverage, spatial resolutions under SMAP are approximately 40 km instead of 100 km with Aquarius. The increased spatial coverage provides opportunities for applying SMAP data for higher resolution studies than Aquarius. With the initiation of SMAP science operations and data flows in April of 2015, the approximate 3-month overlap period between SMAP and Aquarius also allows for inter-calibration and comparative studies.

The primary SMAP salinity datasets include a Level 2 orbital dataset, in which data granules contain both the ascending and descending arcs of the orbit, and two Level 3 gridded datasets: an 8-day running average (linked to the day repeat cycle of SMAP) and monthly average. SMAP SSS data are archived and distributed via the PO.DAAC. SMAP soil moisture and L4 carbon datasets are available from the National Snow and Ice Data Center (NSIDC), with Level 1 SMAP radar data distributed by the Alaska Satellite Facility (ASF). These DAACs are the official NASA repositories for SMAP mission data.

  • Radiometer – L-band microwave radiometer to measure the brightness temperature of microwave emissions from the ocean surface with a center frequency of 1.41 GHz.
  • Radar – L-band (1.26 GHz) radar to measure the echoes of very short radio frequency (RF) pulses that bounce ("backscatter") off the Earth's surface. The radar uses a special technique known as "synthetic aperture" to resolve the RF backscatter over much smaller surface areas that would otherwise be possible.
News and Announcements (chronological order)

JPL SMAP Sea Surface Salinity (SSS) CAP V4.2 Dataset released

Friday, January 25, 2019

The PO.DAAC is pleased to announce the availability of the PI-produced JPL V4.2 SMAP Sea Surface Salinity (SSS) and extreme winds data, which includes Level-2B (L2B) and Level 3 (L3) standard datasets based on the JPL Combined Active-Passive (CAP) algorithm applied to data from the NASA Soil Moisture Active Passive (SMAP) observatory.  Included as part of this release is also a near real-time L2B dataset, available with a latency of about 4.3 hours as a 8-day rolling store.  For science applications, use of the standard products is strongly advised.  We also recommend the use of the V4.2 forward stream data over the prior version, which is deprecated and will be retired within the next 6 months.

SMAP SSS V4.2 accommodates changes in the underlying Level 1B-TB data resulting from the SMAP mission data reprocessing in June 2018. Updates with version 4.2 include: improvement in the brightness temperature (TB) calibration using an adjusted reflector emissivity, inclusion of a Sea Surface Temperature (SST) dependent flat surface emissivity model, use of updated land correction tables, and inclusion of averaged ice concentration ancillary data in the V4.2 datasets.  Data begins on 1 April 2015 and is ongoing.  Data sets comprising this release include the L2B orbital data and two L3 mapped, global coverage salinity datasets: an 8-day running mean dataset based on the repeat orbit of the SMAP mission, along with a monthly average dataset.  The L2B data are provided on a 25 km swath grid while the L3 data are gridded at 0.25o x 0.25o. The spatial resolution of all four datasets is approximately 60 km.  The L2B dataset is in HDF5 format, and the L3 datasets are in netCDF4 file format.  All datasets are CF/ACDD metadata standards compliant, and include estimated SSS uncertainty fields.

SMAP Sea Surface Salinity (SSS) V3.0 Dataset from Remote Sensing Systems (RSS) Released

Monday, November 5, 2018

The PO.DAAC is pleased to announce the availability of the version 3.0 SMAP Sea Surface Salinity (SSS) data from Remote Sensing Systems (RSS).  Several enhancements over the previous release are implemented including:

  • Use of Version 4 L1B SMAP RFI filtered antenna temperatures.
  • Implementation of the geophysical model function from Aquarius version 5 adapted to SMAP.
  • Use of the near real time CCMP wind speed and direction data as ancillary input.
  • Inclusion of IMERG rain rate for the atmospheric liquid cloud water correction and rain flagging.
  • Improved computation of antenna weighted land fraction (gland) and enhanced correction for land radiation intrusion from antenna sidelobes.
  • Improved correction for emissive SMAP mesh antenna.
  • The salty biases at low latitudes and the fresh biases at high N latitudes that were observed in the previous release, are no longer present or are much reduced in V3.0.

Users should note that significant degradation in the performance is observed if the gain weighted land fraction (gland) exceeds 1%. Consequently, observations with gland>0.8% are not used in the Level 3 processing.

Datasets comprising this release include 40 km and 70 km resolution products at level 2C and 3. L3 mapped salinity products consist of an 8-day running mean product based on the repeat orbit of the SMAP mission, along with a monthly average product.  All products are in the netCDF4 file format and are CF/ACDD metadata standards compliant.

For most open ocean applications, the 70-km products are the best to use as they have significantly lower noise than the 40-km products.

See all Official NASA SMAP Mission News
See all PO.DAAC Announcements


RSS SMAP Sea Surface Salinity 70 km 8-Day Global Animation

Animation of global sea surface salinity (SSS) over the period 27 March 2015 to 16 April 2018 based on the 8-day running mean SMAP dataset from Remote Sensing Systems (RSS) at a spatial resolution of 70 km.


RSS SMAP Sea Surface Salinity 70 km 8-Day Animation

Animation of sea surface salinity (SSS) over the period 27 March 2015 to 16 April 2018 based on the 8-day running mean SMAP dataset from Remote Sensing Systems (RSS) at a spatial resolution of 70 km.